The present invention relates to ceramic compositions allowing low-temperature sintering and having a high dielectric constant.
As is well known in the art, ceramic compositions principally consisting of barium titanate (Ba Ti O.sub.3) have been widely used in practice as a dielectric material of high dielectric constant. However, the ceramic compositions principally consisting of Ba Ti O.sub.3 must be sintered at a high temperature in the range from 1300.degree. C. to 1400.degree. C., and hence, they have a shortcoming that in cases where they are used in electronic parts such as, for example, multilayer ceramic capacitors, costly noble metals such as platinum, palladium or the like which can stand such high sintering temperature must be used as internal electrodes of the capacitors.
Accordingly, there has been a strong demand for ceramic dielectric materials which can be sintered at a temperature as low as possible, especially at a temperature below 1000.degree. C. and preferably below 960.degree. C. for enabling the use of inexpensive metals such as nickel, silver or the like as the internal electrodes and yet which have a high dielectric constant.
U.S. Patent 4,078,938 has proposed binary system ceramic compositions consisting of Pb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3 and Pb (Fe.sub.2/3 W.sub.1/3)O.sub.3 and ternary system ceramic compositions consisting of Pb(Fe.sub.2/3 W.sub.1/3)O.sub.3, Pb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3 and Pb(Mn.sub.1/3 Me.sub.170 )O.sub.3 (where Me represents any one of Nb, Ta and Sb), both of which can be sintered at a low temperature. However, the former binary system compositions have a shortcoming that their dielectric loss is relatively large, though their dielectric constant is as high as 8000 or higher at a room temperature.
The latter ternary system compositions have a very high dielectric constant, a low dielectric loss and a high specific resistance where the Pb(Mn.sub.1/3 Me.sub.170 )O.sub.3 content is small. However, the temperature coefficient of the dielectric constant thereof is so large that they are principally only adapted to ceramic capacitors of the Y5V rating of the EIA (U.S. Electronics Industries Association) Standard which means that a temperature coefficient of an electrostatic capacitor lies within a range of +22 to -82% at an operating temperature of -30.degree. to +85.degree. C. For the ceramics of the aforementioned ternary system compositions to meet the EIA Standard's Z5U rating (a temperature coefficient of a capacity of +20 to -56% at an operating temperature ranging +10.degree. to +85.degree. C.), the amount of the Pb(Mn.sub.1/3 Me.sub.2/3)O.sub.3 content would have to be increased. However, an increase in that content will result, where Me is Nb or Ta, in that tan .delta., the dielectric constant, and the specific resistivity of the compositions are deteriorated, as indicated by the Specimen Nos. 48 and 56 in Table 3 of U.S. Pat. No. 4,078,938; or where Me is Sb, in that the dielectric constant and a the specific resistivity are at a low level, though tan .delta. is small, as indicated by the Specimen No. 63 in the same table.
According to the EIA Standard, the insulating resistance of laminated capacitors of Z5U rating is regulated to be 7.5.times.10.sup.9 .OMEGA. or higher, or in terms of a product of capacitance (C) by resistance (R) it is regulated to be 75 F.OMEGA. or higher. In other words, it is regulated as follows: ##EQU1## where .epsilon. is the relative dielectric constant of the dielectric material in the capacitor, .epsilon..sub.o is the dielectric constant of a vacuum, .rho. is the specific resistivity (in .OMEGA..cm), d is the thickness of the dielectric material, and S an area of the dielectric material. In view of this relationship, it is necessary to raise the resistance, if the dielectric constant of the dielectric material is lowered. For example, in the case of .epsilon.=10,000, a dielectric material must have a specific resistivity of 8.47.times.10.sup.10 .OMEGA.cm or higher, whereas in the case of .epsilon.=5000, at least a doubled specific resistivity of 1.69.times.10.sup.11 .OMEGA.cm or higher becomes necessary. So long as a specific resistivity higher than this value is possessed, a capacitor having any capacitance can suffice the requirement for the insulating resistance. However, if a specific resistivity is lower than 1.69.times.10.sup.11 .OMEGA.cm in the case of .epsilon.=5000, then only a laminated capacitor having a capacitance of 0.01 .mu.F. or less is produced. Also, it is required to have a tan .delta. of 2.5% or less in the Z5U rating. In conclusion, with the ternary system compositions disclosed in U.S. Pat. No. 4,078,938, it is impossible to produce a laminated capacitor adapted to the Z5U rating capacitors in the practical capacitance range even by increasing the amount of the Pb (Mn.sub.1/3 Me.sub.170 )O.sub.3 content. Much less, laminated capacitors adapted to the Z5T rating (the temperature coefficient of the capacitance being .+-.10% in the temperature range of 10.degree. to +85.degree. C.), the Z5S rating (the temperature coefficient of the capacitance being .+-.22% in the same temperature range) or the Z5R rating (the temperature coefficient of the capacitance being .+-.15% in the same temperature range) all of the EIA Standard cannot be produced.
Also proposed have been binary system ceramic compositions consisting of Pb(Fe.sub.170 W.sub.1/3)O.sub.3 and Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3, as disclosed in patent application No. 53-19600 published before examination. However, with this binary system composition, though the specific resistance is increased as the proportion of Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 is increased, the dielectric constant reaches the maximum where Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 is 40 mol % and decreases as it is further increased. Consequently, Z5U rating capacitors cannot be produced by use of these compositions. Moreover, tan .delta., the temperature coefficient of the dielectric constant and the specific resistivity of these compositions are liable to vary depending upon a sintering temperature, and therefore, mass-production of capacitors is difficult.